CD4 and systemic lupus erythematosus: Using a CD137 ligand (CD137L) knockout B6.MRL.lpr−/− spontaneous murine SLE model, our group has recently demonstrated that while the CD4+Tbet+ (Th1) subset is increased in the B6.MRL.lpr−/− mice, the absence of the co-stimulatory molecule CD137L increases the severity of SLE via two pathways [50].